chore(draw): expose extension points

+ clearly documented as *UNSAFE*
+ add test cases
+ tested existing *UNSAFE* handler endpoint

lib/draw/BpmnRenderer.js

@@ -382,16 +382,16 @@ export default function BpmnRenderer(
     return drawPath(parentGfx, path, assign({ 'data-marker': type }, attrs));
   }
 
-  function as(type) {
-    return function(parentGfx, element) {
-      return handlers[type](parentGfx, element);
-    };
-  }
-
   function renderer(type) {
     return handlers[type];
   }
 
+  function as(type) {
+    return function(parentGfx, element) {
+      return renderer(type)(parentGfx, element);
+    };
+  }
+
   function renderEventContent(element, parentGfx) {
 
     var event = getSemantic(element);
@@ -1872,6 +1872,7 @@ export default function BpmnRenderer(
   // extension API, use at your own risk
   this._drawPath = drawPath;
 
+  this._renderer = renderer;
 }
 
 
@@ -1893,7 +1894,7 @@ BpmnRenderer.prototype.canRender = function(element) {
 
 BpmnRenderer.prototype.drawShape = function(parentGfx, element) {
   var type = element.type;
-  var h = this.handlers[type];
+  var h = this._renderer(type);
 
   /* jshint -W040 */
   return h(parentGfx, element);
@@ -1901,7 +1902,7 @@ BpmnRenderer.prototype.drawShape = function(parentGfx, element) {
 
 BpmnRenderer.prototype.drawConnection = function(parentGfx, element) {
   var type = element.type;
-  var h = this.handlers[type];
+  var h = this._renderer(type);
 
   /* jshint -W040 */
   return h(parentGfx, element);

test/spec/draw/BpmnRendererSpec.js

@@ -671,8 +671,13 @@ describe('draw - bpmn renderer', function() {
     it('should expose helpers', inject(function(bpmnRenderer) {
 
       // then
-      expect(bpmnRenderer._drawPath).to.be.a('function');
 
+      // unsafe to use render APIs
+      expect(bpmnRenderer._drawPath).to.be.a('function');
+      expect(bpmnRenderer._renderer).to.be.a('function');
+
+      // very unsafe to use internal state
+      expect(bpmnRenderer.handlers).to.exist;
     }));
 
   });